Knowing how much to eliminate depends on the amount of living space you are going to downsize. Before we moved into our current house, the wife, kids and I lived in 200 square feet per person and besides sharing one bathroom we didn't have many problems in this space. From experience, less space promotes more outdoors living. The tiny house movement as a general rule cites about 100 square feet per person while the 80/20 rule suggests that we'd be limited to 480 square feet. Starting with a base of 200 square feet per person plus a standard square footage for bath room(s) and kitchen, we came up with 480 square feet as the base. We also wanted a master bedroom/bath which added another 288 sq ft which gave us a total of 768 square feet. This equals about 33% of our current house and as we use less than half of our house it seems pretty close. This is how we came to a 2/3 or 66% reduction in belongings.
"As you simplity your life, the laws of the universe will be simpler; solitude will not be solitude, poverty will not be poverty, nor weakness weakness." Henry David Thoreau